CONSTRAINT ON THE GEOCHEMICAL STAGE CAUSING TETRAD EFFECT IN KIMURAITE - COMPARATIVE-STUDIES ON KIMURAITE AND ITS RELATED ROCKS, FROM REE PATTERN AND ND ISOTOPE RATIO

Our careful analysis (Akagi et al., 1993) disclosed a W type tetrad ef fect in kimuraite, an REE carbonate mineral. To put constraint on the genetic stage responsible for the appearance of the effect, rare earth element (REE) abundances were compared between kimuraite and its para genetic mineral (lanthanite) and between kimuraite and its host rock. The Nd isotopic compositions of kimuraite and the host rock were also compared. Neodymium isotopic analyses conclude that the REE in kimurai te has a different source to its host rock. The similar magnitude of t he tetrad effect among the paragenetic minerals of kimuraite rules out the process of mineralization as the stage responsible for the tetrad effect. It is inferred that kimuraite and lanthanite formed in situ f rom aqueous solutions enriched in REE which had infiltered the fissure s of the host rock. This solution is considered to be responsible prim arily for the tetrad effect of kimuraite.